Well the biggest disadvantage is that only one of you will have one. So while one of you may want to go to a park, the other may not want to because of wasting a day on their ticket or one may not be able to go because they don't have any days left.
Advantages are unlimted park admission for a year. And really that is all that should be looked at.
Sure there are discounts and perks but they aren't guaranteed and are subject to change at any time.
Room discounts for example used to be much better with an AP rate, they were a % off. Now they are a flat dollar amount off and from what I have been seeing they are fewer discounted rooms avail and the dollar amount is becoming less over time.
So its a gamble if you want to go that route.
Also last year there were many more ticket discounts (which was the first time they offered them) for
MVMCP. This year there are only 4 dates.
So while the perks are nice, they are just perks and really should only be looked at for park admission.
Another disadvantage is that they burn a whole in your pocket. Its easy to want to take a small trip because you can and have admission.